1. Field of the Invention
The present invention relates to an off-center adjustment apparatus in which registration rollers and pre-registration rollers are provided upstream from an electrostatic latent image carrier, a front end of transported paper is seized by the registration rollers and a rear end thereof is seized by the pre-registration rollers so as to temporarily stop the paper, and in this state, the registration rollers are moved in a direction perpendicular to a paper transport direction, thereby performing center position adjustment in the direction perpendicular to the paper transport direction.
2. Description of the Related Art
In recent years, longitudinal transport type image forming apparatuses have become the mainstream because they require less installation space. Specifically, a plurality of paper feed cassettes stacked vertically are provided in a lower portion of the main body of the apparatus, and a transfer portion and a fixing portion are provided in an upper portion of the main body. In such a structure, transported paper fed from the paper feed cassette is temporarily transported upward and, thereafter, is bent by about 90 degrees before the transfer portion so that the transport direction is converted into a horizontal direction before being transported toward the transfer portion.
FIG. 6 shows a configuration of a paper transport path before the transfer portion.
The paper transport path 28, which is bent as described above, comprises a pair of transport rollers R31 and R32, a pair of pre-registration rollers R41 and R42, and a pair of registration rollers R51 and R52, which are arranged sequentially along a paper transport direction S. The registration rollers R51 and R52 are disposed at a distance of about 50 mm away from a nip portion N1 where an electrostatic latent image carrier (photosensitive drum) 14 and a transfer roller 17a contact each other.
FIGS. 7(a) and 7(b) are a chart showing operation timing of the registration rollers R51 and R52 and the pre-registration rollers R41 and R42.
Specifically, when paper P is transported on the paper transport path 28 to the registration rollers R51 and R52 and the front end of the paper P comes in contact with a nip portion N5 between the registration rollers R51 and R52, the registration rollers R51 and R52 first stop at time t1, and the pre-registration rollers R41 and R42 then stop at slightly later time t2. By introduction of such a difference between the stop times, the paper P seized by the registration rollers R51 and R52 and the pre-registration rollers R41 and R42 is held while being slightly bent or curved as shown in FIG. 6.
Thereafter, the registration rollers R51 and R52 and the pre-registration rollers R41 and R42 resume transportation of the paper P at time t3 which leads to timing with which the front end of the paper P coincides with a front end of image information developed on the electrostatic latent image carrier (photosensitive drum) 14. In this case, since the paper P is slightly bent, the timing of outputting the front end of the paper P from the registration rollers R51 and R52 can be caused to be uniform in the width direction, so that the paper P is prevented from being obliquely transported, thereby eliminating a distortion in a formed image with respect to the width direction of the paper P. Specifically, the paper P is temporarily stopped at the registration rollers R51 and R52, and while the paper P is stopped, the registration rollers R51 and R52 are used to finely adjust a position of the front end of the paper P in a direction parallel to the paper transport direction S, adjust a center position of the transported paper, correct the obliqueness of the paper P during transportation, or the like.
In such adjustment by the registration rollers R51 and R52, the center position adjustment of the transported paper is performed as follows. The amount of displacement of a center of the transported paper is detected by a detection means, such as a line sensor or the like. The registration rollers R51 and R52 are shifted by an amount corresponding to the detected amount in a direction perpendicular to the paper transport direction. Since the front end portion of the transported paper is seized (chucked) by the registration rollers R51 and R52, the movement is smoothly performed. However, the rear end portion of the transported paper is seized (chucked) by the pre-registration rollers R41 and R42, which basically do not have a movement mechanism, so that a torsion phenomenon occurs in the paper seized by the registration rollers R51 and R52 and the pre-registration rollers R41 and R42, and therefore, a wrinkle occurs when transportation is resumed.
To solve such a problem, a technique has been proposed (e.g., JP 2005-67806A).
JP 2005-67806A describes an image forming apparatus in which one (drive roller) of transport rollers immediately before registration rollers is divided into a plurality of drive roller segments. Of these segments, a pair of drive roller segments provided at laterally symmetrical positions about a paper feed center line have laterally symmetrical taper shapes which increase in thickness toward the paper feed center line, i.e., decrease in thickness toward the opposite direction. With such a configuration, when the shaft is rotated, one of the drive roller segments tends to more greatly advance the paper at a portion closer to the paper feed center line, so that a portion of the paper contacting the drive roller segment tends to turn to the left. The other drive roller segment also tends to more greatly advance a portion closer to the paper feed center line, however, the other drive roller segment has the mirror reversed shape of the one drive roller segment, so that a portion of the paper contacting the other drive roller segment tends to turn to the right. These motions take place simultaneously, so that the paper is advanced forward in a manner which smoothes a wrinkle.
According to the image forming apparatus of JP 2005-67806A described above, it is possible to reliably smooth a wrinkle of paper. However, when the torsion phenomenon occurs due to the center position adjustment by the registration rollers, a wrinkle occurs due to the torsion phenomenon when transportation is resumed, because the transport rollers (corresponding to pre-registration rollers of the present invention) immediately before the registration rollers are fixed in a direction perpendicular to the paper transport direction.